In the field of moving image coding technology, encoding is generally carried out in the following manner. That is, image data (image information) is segmented into prediction blocks, prediction is carried out in prediction blocks thus segmented, orthogonal transform is carried out with respect to difference (residual) between a predicted image and an input image to obtain a transform coefficient, the transform coefficient thus obtained is quantized, and then encoding is made by variable-length coding of the transform coefficient thus quantized. Orthogonal transform of the difference (residual) is carried out in transform blocks, which are identical to the prediction blocks or are determined independently from the prediction blocks. With such an encoding method, great distortion (so-called black noise) is likely to occur at a boundary of each block, which is a unit for prediction or a unit for orthogonal transform. In addition, encoding can cause not only block noise but also various encoding noise such as blurring and mosquito noise.
In order to reduce these distortions, there have been disclosed techniques of performing filtering operation with respect to a decoded image used as a reference image at the prediction.
Non-Patent Literature 1 utilizes the technique of changing a strength of block noise reduction process in accordance with a threshold that can be embedded into coded data.
Non-Patent Literature 2 discloses the technique called Adaptive Loop Filter (ALF), by which pixels (5×5, 7×7, or 9×9) around a filter target pixel are referred to, and weighting factors of these pixels thus referred are decoded so that a spatial filtering operation is carried out. The ALF is a technique of determining, for each slice on a reference image, a weighting factor that would minimize an error between a reference image subjected to filtering and an original image, and subjecting to filtering operation based on weighting factor.
Furthermore, Non-Patent Literature 3 discloses a configuration in which a prediction residual, a prediction image, and a decoded image (sum of the prediction residual and the prediction image) applied to non-linear de-noising filter is regarded as an input image of ALF, and in which a de-blocking filter is provided at a stage subsequent to the ALF.